Polymer solar cells with bulk heterojunction (BHJ) active layers have attracted significant research efforts due to their large surface to volume ratio, tunability, processability, and potential low-cost. The structure and property of the components in the BHJ are investigated and optimized in order to produce appreciable power conversion efficiency (PCE). While research on p-type conjugated polymers has focused largely on reducing band gaps for the absorption of a larger fraction of the solar spectrum, many n-type acceptors, including small molecules and fullerenes, have been developed to be electronically and morphologically compatible with their p-type counterparts. Covalently functionalized fullerenes such as PCBM ([6,6]-phenyl-C61-butyric acid methyl ester) and ICBA (Indene-C60 Bisadducts) are widely used n-type molecules and have been shown to generate promising PCEs in combination with suitable polymer counterparts. In particular, notably high open-circuit voltages (VOC) of P3HT:ICBA BHJ solar cell have been observed and attributed to the high-lying LUMO level of ICBA, since a VOC of an organic solar cell is considered to have linear correlation with an effective band gap, HOMOD-LUMOA (A: acceptor, D: donor). Hence, novel acceptors with high LUMO levels have been designed and investigated for high VOC devices.
Most of the covalent functionalization methods applied on fullerenes generally decrease their electron affinity and raise the LUMO levels by breaking the full conjugation of the fullerene surface. However, the extent of the change can depend on the structural and electronic property of the addend, as seen when comparing different fullerene derivatives. In previous studies of tetraalkylcyclobutadiene-C60 adducts, the cofacial interaction between C60 and π-orbital of the addend was shown to induce a significant change of the LUMO energy levels of fullerenes. Indene-C60 adducts also exhibited considerable cofacial interactions due to the aromatic moiety near the C60 surface.